docker: Raise UDP buffer size in example (#7536)

README-Docker.md

@@ -13,7 +13,8 @@ altered with the ``PUID`` and ``PGID`` environment variables.
 
 ```
 $ docker pull syncthing/syncthing
-$ docker run -p 8384:8384 -p 22000:22000/tcp -p 22000:22000/udp \
+$ docker run --sysctl net.core.rmem_max=2097152 \
+    -p 8384:8384 -p 22000:22000/tcp -p 22000:22000/udp \
     -v /wherever/st-sync:/var/syncthing \
     syncthing/syncthing:latest
 ```